Just went through this last month, couldn't get any heat out of the forge, come to find out that the shop is fed with 1/2" line from the meter, even though it was at 2 psi, the flow wasn't there, switched over to 1 1/4" line and the gas guy says that it is now capable of supplying 1,000,000 btu's to the shop, where as before it was only like 100,000. If you call the NG comp they will ask how far from the meter and what size line do you have and they have a chart that will estimate the btu output, I needed 450,000 as it is stamped on the forge, I wasn't anywhere near that.
Love this site, been a lurker for a while just figured I could maybe help with your answer.